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Abstract:

A firing head embodiment of the invention confines a connected
capacitance cartridge, explosive detonator, and wireline connection
switch within an independent, cylindrical housing tube that is
environmentally capped at both ends by threaded closures for secure
transport to a well site.

Claims:

1. A method of arming an explosive well tool comprising the steps of:
providing an axially elongated firing head housing having first
mechanical assembly means at one end thereof: securing an explosive
detonator means to said housing to project axially beyond said one end of
said housing; enclosing said detonator means by a detonator cover of
sufficient structural integrity to confine a detonation of said detonator
means; securing said detonator cover to said housing by said first
assembly means: providing operating signal contact means at an opposite
axial end of said housing; providing a capacitive firing cartridge
connected electrically between said signal contacts and said detonator
means; delivering said housing to a well site with said firing cartridge
connected to said signal contact means and to said detonator means and
said detonator cover enclosing said detonator means and secured to said
first assembly means; at said well site, with said detonator cover
secured to said first assembly means, securing signal carrier means to
said opposite end of said housing for signal continuity with said signal
contact means; removing said detonator cover means from said housing;
and, connecting an explosive well tool to said housing at said one end by
said first assembly means.

2. A method of arming an explosive well tool as described by claim 1
wherein said explosive well tool connection comprises the step of
connecting a shaped charge tubing cutter.

3. A method of arming an explosive well tool as described by claim 1
further comprising the steps of: providing environmental cover means for
said signal contact means; providing said housing with second mechanical
assembly means proximate of said housing opposite end; securing said
environmental cover to said housing by said second assembly means;
delivering said housing to a well site with said environmental cover
secured to said second assembly means; removing said environmental cover
from said housing; and connecting said signal carrier means with said
second assembly means.

4. A method of arming an explosive well tool as described by claim 1
wherein said first assembly means is provided with screw threads.\

5. A method of arming an explosive well tool as described by claim 1
wherein said second assembly means is provided with screw threads.

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Division of presently pending application
Ser. No. 13/317,657 filed Oct. 25, 2011. Said application Ser. No.
13/317,657 is a Continuation of U.S. Pat. No. 8,136,439.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to the earthboring arts. More
particularly, the invention relates to methods and devices for severing
drill pipe, casing and other massive tubular structures by the remote
detonation of an explosive cutting charge.

[0005] 2. Description of Related Art

[0006] Deep well earthboring for gas, crude petroleum, minerals and even
water or steam requires tubes of massive size and wall thickness. Tubular
drill strings may be suspended into a borehole that penetrates the
earth's crust several miles beneath the drilling platform at the earth's
surface. To further complicate matters, the borehole may be turned to a
more horizontal course to follow a stratification plane.

[0007] The operational circumstances of such industrial enterprise
occasionally presents a driller with a catastrophe that requires him to
sever his pipe string at a point deep within the wellbore. For example, a
great length of wellbore sidewall may collapse against the drill string
causing it to wedge tightly in the well bore. The drill string cannot be
pulled from the well bore and in many cases, cannot even be rotated. A
typical response for salvaging the borehole investment is to sever the
drill string above the obstruction, withdraw the freed drill string above
the obstruction and return with a "fishing" tool to free and remove the
wedged portion of drill string.

[0008] When an operational event such as a "stuck" drill string occurs,
the driller may use wireline suspended instrumentation that is lowered
within the central, drill pipe flow bore to locate and measure the depth
position of the obstruction. This information may be used to thereafter
position an explosive severing tool within the drill pipe flow bore.

[0009] Typically, an explosive drill pipe severing tool comprises a
significant quantity, 800 to 1,500 grams for example, of high order
explosive such as RDX, HMX or HNS. The explosive powder is compacted into
high density "pellets" of about 22.7 to about 38 grams each. The pellet
density is compacted to about 1.6 to about 1.65 gms/cm3 to achieve a
shock wave velocity greater than about 30,000 ft/sec, for example. A
shock wave of such magnitude provides a pulse of pressure in the order of
4×106 psi. It is the pressure pulse that severs the pipe.

[0010] In one form, the pellets are compacted at a production facility
into a cylindrical shape for serial, juxtaposed loading at the jobsite as
a column in a cylindrical barrel of a tool cartridge. Due to weight
variations within an acceptable range of tolerance between individual
pellets, the axial length of explosive pellets fluctuates within a known
tolerance range. Furthermore, the diameter-to-axial length ratio of the
pellets is such that allows some pellets to wedge in the tool cartridge
barrel when loaded. For this reason, a go-no-go type of plug gauge is
used by the prior art at the end of a barrel to verify the number of
pellets in the tool barrel. In the frequent event that the tool must be
disarmed, the pellets may also wedge in the barrel upon removal. A
non-sparking depth-rod is inserted down the tool barrel to verify removal
of all pellets.

[0011] Extreme well depth is often accompanied by extreme hydrostatic
pressure. Hence, the drill string severing operation may need to be
executed at 10,000 to 20,000 psi. Such high hydrostatic pressures tend to
attenuate and suppress the pressure of an explosive pulse to such degree
as to prevent separation.

[0012] One prior effort by the industry to enhance the pipe severing
pressure pulse and overcome high hydrostatic pressure suppression has
been to detonate the explosive pellet column at both ends simultaneously.
Theoretically, simultaneous detonations at opposite ends of the pellet
column will provide a shock front from one end colliding with the shock
front from the opposite end within the pellet column at the center of the
column length. On collision, the pressure is multiplied, at the point of
collision, by about 4 to 5 times the normal pressure cited above. To
achieve this result, however, the detonation process, particularly the
simultaneous firing of the detonators, must be timed precisely in order
to assure collision within the explosive column at the center.

[0013] Such precise timing is typically provided by means of mild
detonating fuse and special boosters. However, if fuse length is not
accurate or problems exist in the booster/detonator connections, the
collision may not be realized at all and the device will operate as a
"non-colliding" tool with substantially reduced severing pressures.

[0014] The reliability of prior art severing tools is further compromised
by complex assembly and arming procedures required at the well site. Laws
and regulations require that explosive components (detonator, pellets,
etc.) must be transported separately from the tool body. Complete
assembly must take place at the well site. Unfortunately, such final
assembly is often undertaken in unfavorable working conditions.

[0016] An alternative embodiment of the invention that is particularly
well suited for single point ignition provides a unitized firing head
that is severable from an explosive housing for separate and independent
transport to a well site.

SUMMARY OF THE INVENTION

[0017] The pipe severing tool of the present invention comprises an outer
housing that is a thin wall metallic tube of such outside diameter that
is compatible with the drill pipe flow bore diameter intended for use.
The upper end of the housing tube is sealed with a threaded plug having
insulated electrical connectors along an axial aperture. The housing
upper end plug is externally prepared to receive the intended suspension
string such as an electrically conductive wireline bail or a continuous
tubing connecting sub.

[0018] The lower end of the outer housing tube is closed with a tubular
assembly that includes a stab fit nose plug. The nose plug assembly
includes a relatively short length of heavy wall tube extending axially
out from an internal bore plug. The bore plug penetrates the barrel of
the housing tube end whereas the tubular portion of the nose plug extends
from the lower end of the housing tube. The bore plug is perimeter sealed
by high pressure O-rings and secured by a plurality of set screws around
the outside diameter of the outer housing tube.

[0019] The tubular portion of the nose plug provides a closed chamber
space for enclosing electrical conductors. The bore plug includes a
tubular aperture along the nose plug axis that is a load rod alignment
guide. Laterally of the load rod alignment guide is a socket for an
exploding bridge wire (EBW) detonator or an exploding foil initiator
(EFI).

[0020] Within the upper end of the outer housing barrel is an inner
tubular housing for an electronic detonation cartridge having a
relatively high discharge voltage, 5,000 v or more, for example. Below
the inner tubular housing is a cylindrical, upper detonator housing. The
upper detonator housing is resiliently separated from the lower end of
the inner tubular housing by a suitable spring. The upper detonator
housing includes a receptacle socket 31 for an exploding bridge wire
(EBW) detonator. The axis for the upper detonator receptacle socket is
laterally offset from the outer housing barrel axis.

[0021] Preferably, the severing tool structure is transported to a working
location in a primed condition with upper and lower EBW detonators
connected for firing but having no high explosive pellets placed between
the EBW detonators. At the appropriate moment, the nose plug assembly is
removed from the bottom end of the outer housing and a load rod therein
removed. The upper distal end of the load rod includes a circumferential
collar such as a snap ring. The opposite end of the load rod is visually
marked to designate maximum and minimum quantities of explosive aligned
along the load rod.

[0022] Explosive pellets for the invention are formed as solid cylinder
sections having an axial aperture. The individual pellets are stacked
along the load rod with the load rod penetrating the axial aperture. The
upper distal end collar serves as a stop limit for the pellets which are
serially aligned along the rod until the lower face of the lowermost
pellet coincides with the max/min indicia marking. A restriction collar
such as a resilient O-ring is placed around the loading rod and tightly
against the bottom face of the lowermost explosive pellet.

[0023] The rod and pellet assembly are inserted into the outer housing
barrel until the uppermost pellet face contiguously engages the upper
detonator housing. The rod guide aperture in the nose plug is then
assembled over the lower distal end of the load rod and the lower
detonator brought into contiguous engagement with the lowermost pellet
face. The assembly is then further compressed against the loading spring
between the inner tubular housing and the upper detonator housing until
abutment between the nose plug shoulder and the lower distal end of the
outer housing tube.

[0024] In the event that the invention severing tool must be disarmed, all
pellets may be removed from the housing barrel as a singular unit about
the load rod. This is accomplished by removing the lower nose plug which
exposes the lower end of the load rod. By grasping and pulling the load
rod from the housing barrel, all pellets that are pinned along the load
rod below the upper distal end collar are drawn out of the housing tube
with the rod.

[0025] An alternative embodiment of the invention consolidates all of the
explosive ignition components into a closed cylinder that is
independently packaged and transported.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Relative to the drawings wherein like reference characters
designate like or similar elements or steps through the several figures
of the drawings:

[0027] FIG. 1 is a sectional view of the invention as assembled without an
explosive charge for transport;

[0028]FIG. 2 is a sectional view of the invention with the bottom nose
piece detached from the main assembly housing;

[0029]FIG. 3 is a sectional view of an assembled, explosive pellet unit;

[0030]FIG. 4 is a sectional view of the invention with the explosive
pellet unit combined with the main assembly housing but the bottom nose
piece detached therefrom;

[0031]FIG. 5 is a sectional view of the invention in operative assembly
with an explosive pellet unit.

[0032]FIG. 6 is an alternative embodiment of the invention illustrating
an independently transported firing head.

[0033]FIG. 7 illustrates a state of arrival for the firing head in a tool
arming sequence.

[0039] Referring to the FIG. 1 cross-sectional view of the invention 10, a
tubular outer housing 12 having an internal bore 14 is sealed at an upper
end by a plug 16. The plug 16 includes an axial bore 18 and an electrical
connector 20 for routing detonation signal leads 22. A boss 17,
projecting from the base of the plug, is externally threaded for the
attachment of the desired suspension string such as an electrical
wireline or service tubing.

[0040] An inner housing tube 24 is secured to and extends from the upper
end plug 16 into the internal bore 14 of the outer housing 12. The inner
housing tube 24 encloses a capacitive firing cartridge 26. Below the
inner housing 24 is an upper detonator housing 28. A coil spring 30 links
the upper detonator housing 28 to the inner housing tube 24. An exploding
bridge wire (EBW) detonator or exploding foil initiator (EFI) 32 is
seated within a receptacle socket formed in the upper detonator housing
28 laterally of the housing axis. Electrical conduits 34 connect the
capacitive firing cartridge 26 to the EBW detonator or EFI 32.

[0041] An exploding bridge wire (EBW) detonator comprises a small quantity
of moderate to high order explosive that is detonated by the explosive
vaporization of a metal filament or foil (EFI) due to a high voltage
surge imposed upon the filament. A capacitive firing cartridge is
basically an electrical capacitor discharge circuit that functions to
abruptly discharge with a high threshold voltage. Significantly, the EBW
detonator or EFI is relatively insensitive to static or RF frequency
voltages. Consequently, the capacitive firing circuit and EBW or EFI
function cooperatively to provide a substantial safety advantage. An
unusually high voltage surge is required to detonate the EBW detonator
(or EFI) and the capacitive firing cartridge delivers the high voltage
surge in a precisely controlled manner. The system is relatively
impervious to static discharges, stray electrical fields and radio
frequency emissions. Since the EBW and EFI detonation systems are,
functionally, the same, hereafter and in the attached invention claims,
reference to an EBW detonator is intended to include and encompass an
EFI.

[0042] The lower end of the outer housing tube 12 is operatively opened
and closed by a nose plug 40. The nose plug 40 comprises a plug base 42
having an O-ring fitting within the lower end of the outer housing bore
14. The plug base 42 may be secured to the outer housing tube 12 by shear
pins or screws 44 to accommodate a straight push assembly. Projecting
from the interior end of the plug base is a guide tube boss 46 having an
axial throughbore 48 and a receptacle socket 50 for a detonator cap 66.

[0043] Projecting from the exterior end of the plug base 42 is a heavy
wall nose tube 52 having a nose cap 54. The nose cap 54 may be
disassembled from the nose tube 52 for manual access into the interior
bore 56 of the nose tube 52. Detonation signal conductor leads 58 are
routed from the firing cartridge 26, through the upper detonator housing
and along the wall of housing bore 14. A conductor channel 60 routes the
leads 58 through the nose plug base 42 into the nose tube interior 56.
This nose tube interior provides environmental protection for electrical
connections 62 with conductor leads 64 from the lower EBW detonator 66.

[0044] Although the electrical connections of both EBW detonators 32 and
66 are field accessible, it is a design intent for the invention to
obviate the need for field connections. Without explosive pellet material
in the outer housing bore 14, EBW detonators 32 and 66 are the only
explosive material in the assembly. Moreover, the separation distance
between the EBW detonators 32 and 66 essentially eliminates the
possibility of a sympathetic detonation of the two detonators.
Consequently, without explosive material in the tubing bore 14, the
assembly as illustrated by FIG. 1 is safe for transport with the EBW
detonators 32 and 66 connected in place.

[0045] The significance of having a severing tool that requires no
detonator connections at the well site for arming cannot be minimized.
Severing tools are loaded with high explosive at the well site of use.
Often, this is not an environment that contributes to the focused,
intellectual concentration that the hazardous task requires. Exacerbating
the physical discomfort is the emotional distraction arising from the
apprehension of intimately manipulating a deadly quantity of highly
explosive material. Hence, the well site arming procedure should be as
simple and error-proof as possible. Complete elimination of all
electrical connection steps is most desirable.

[0046] The load rod 70, best illustrated by FIGS. 2, 3 and 4, is
preferably a stiff, slender shaft having an end retainer 72 such as a "C"
clip or snap ring. Preferably, the shaft is fabricated from a
non-sparking material such as wood, glass composite or non-ferrous metal.
Individual high explosive "pellets" 74 are cylindrically formed with a
substantially uniform outer perimeter OD and a substantially uniform ID
center bore. The term "pellets" as used herein is intended to encompass
all appropriate forms of explosive material regardless of the descriptive
label applied such as "cookies", "wafers", or "charges". The axial length
of the pellets may vary within known limits, depending on the exact
weight quantity allocated to a specific pellet. The pellets are assembled
as a serial column over the rod 70 which penetrates the pellet center
bore. A prior calculation has determined the maximum and minimum
cumulative column length depending on the known weight variations. This
maximum and minimum column length is translated onto the rod 70 as an
indicia band 76. The maximum and minimum length dimensions are measured
from the rod end retainer 72. The OD of the end retainer 72 is selected
to be substantially greater than the ID of the pellet center bore. Hence
the pellets cannot pass over the end retainer and can slide along the rod
70 length no further than the end retainer. When loading the tool with
explosive in the field, the correct quantity of explosive 74 will
terminate with a lower end plane that coincides within the indicia band
76. An elastomer O-ring 78 constricted about the shaft of rod 70
compactly confines the pellet assembly along the rod length.

[0047] A lower distal end portion 79 of the rod extends beyond the indicia
band 76 to penetrate the guide bore 48 of the bore plug base 42 when the
bottom nose plug 40 is replaced after an explosive charge has been
positioned. This rod extension allows the high explosive to be manually
manipulated as a singular, integrated unit. In full visual field, the
explosive charge is assembled by a columned alignment of the pellets over
the penetrating length of the rod. When the outside surface plane of the
last pellet in the column aligns within the indicia band 76, the lower
end retainer 78 is positioned over the rod and against the last pellet
surface plane to hold the column in tight, serial assembly. Using the rod
extension 79 as a handle, the explosive assembly is axially inserted into
the housing bore 14 until contiguous contact is made with the lower face
of the upper detonator housing 28.

[0048] One of the synergistic advantages to the unitary rod loading system
of the invention is use of lighter, axially shorter pellets, i.e. 22.7
gms. These lighter weight pellets enjoy a more favorable shipping
classification (UN 1.4S) than that imposed on heavier, 38 gm pellets (UN
1.4D). In a prior art severing tool, the lighter weight pellets would be
avoided due to "cocking" in the tool barrel 14 during loading. The
loading rod system of the present invention substantially eliminates the
"cocking" problem, regardless of how thin the pellet may be.

[0049] With the explosive assembly in place, the lower end of the housing
is closed by placement of the nose plug 40 into the open end of the
housing. The rod end projection 79 penetrates the guide bore 48 as the
plug base 42 is pushed to an internal seal with the housing bore 14. To
assure intimate contact of the opposite end EBW detonators 32 and 66 with
the respective adjacent ends of the explosive assembly, the upper
detonator housing 28 is displaced against the spring 30 to accommodate
the specified length of the explosive column. Accordingly, when the nose
plug 40 is seated against the end of the outer housing tube 12, both EBW
detonators are in oppositely mutual compression as is illustrated by FIG.
5. The severing tool is now prepared for lowering into a well for the
pipe cutting objective

[0050] Presently applied Explosive Safety Recommendations require the
severing tool 10 to be electrically connected to the suspension string
i.e. wireline, etc., before arming ballistically. Ballistic arming with
respect to the present invention means the insertion of the explosive
Pellets 24 into the housing bore 14.

[0051] On those occasions when the severing tool must be disarmed without
discharge, it is only necessary to remove the nose plug 40 and by
grasping the rod extension 79, draw the pellets 74 from the tube bore 14
as a single, integrated item.

[0052] An alternative embodiment of the invention, illustrated by FIG. 6,
represents an independent firing head tool section 80 wherein all of the
explosive initiation components are integrated as a transportable unit
separate from the major tool explosive. The independent firing head 80
externally comprises a housing tube 82 that is fitted with removable end
caps 84 and 90 that protect and environmentally seal the internal
components.

[0053] The upper end cap 84 may be secured by an assembly mechanism such
as screw threads 85 internally of the housing tube bore 100 that begin
axially from an O-ring seal face 86. The end cap 84 may be a closed plug
having corresponding external screw threads 85 leading an O-ring channel
87. Preferably, the internal threads 85 are compatible with external
screw threads of a wireline signal sub or other means by which the
assembled downhole tool is suspended and actuated.

[0054] The lower end cap 90 also is a closed plug having a deep internal
bore 92. The internal bore opening may be provided with an O-ring seal
surface 96 followed axially by assembly means such as internal threads
95.

[0055] In a presently preferred design of the firing head 80, the main
housing tube includes a primary bore 100 of a first internal diameter
extending from the upper end threads 85 to an annular abutment end 102. A
secondary bore 104 extends from the abutment 102 to the lower end of the
tube 82. The lower distal end 104 of the housing 82 forms a socket boss
104 that is externally seized to receive the internal bore of detonator
retainer 106. A cylindrical projection from the base of the detonation
retainer 106 provides a detonator socket 107 for securing the position of
a detonator element 108 such as a Pacific Scientific EBW Part No.
2-300180.

[0056] External threads 95 for the lower end cap 90 extend from the base
of the socket boss 104 to an O-ring 96 channel.

[0057] The axial space within the housing 82 for secure confinement of
electronic components is preferably defined between the annular abutment
102 and an internal snap ring 105. Spacing cylinders 110 and 112 of
nonconductive materials such as plastic or elastomer isolate and axially
confine a capacitor firing cartridge 114 such as the PX-1 fireset by
Ecoss, Inc. of Houston, Tex. within the primary bore 100.

[0058] At the upper end of the electronic assembly within the primary bore
100 between the snap ring 105 and the upper end of spacer 110 is an
electrical contact plug 116 of non-conductive material. Embedded within
the plug 116 is an electrically conductive ground surface 117
electrically connected to a ground terminal pin 118. A resilient contact
pin 119, preferably positioned along the bore axis, passes axially
through the plug 116. Electrically conductive leads 120 and 122 connect
the ground surface 117 and resilient contact 119 to the capacitor firing
cartridge 114. Electrically conductive discharge leads 124 and 126
connect the firing cartridge 114 to the detonator 108.

[0059] In application, the firing head 80 is delivered to a well head in
independent crating or packaging with the end caps 84 and 90 secured in
place by meshing threads, for example, as represented by FIG. 7. Also,
the firing cartridge 114 is electrically connected to the terminal pin
118 and resilient contact 119. Additionally, the firing cartridge
discharge leads 124 and 126 are connected to a socket mounted detonator
108.

[0060] Upon removal from the transport crating, the upper end cap 84 is
removed to expose the internal upper threads 85 as shown by FIG. 8. With
the end cap 85 removed, a wireline signal sub 130 is attached with a
connection adapter 131. This assembly of signal sub 130 engages the
wireline carried signal conductors with the ground surface 117 and
resilient contact 119 for electrical continuity with the firing cartridge
114. Notably, the end cap 90 has remained in place throughout the
wireline connection procedures as shown by FIG. 9 to safely confine any
accidental or unintended discharge of the detonator 108. To this end and
objective, those of ordinary skill will understand that the end cap 90
should be constructed with sufficient structural integrity to confine an
unintended discharge of the detonator 108. Obviously, the type and
quantity of explosive used to compound the detonator 108 will determine
the parameters of structural sufficiency for the end cap 90.

[0061] At this point, the lower end cap 90 is removed to expose the
external screw threads 95 and detonator 108 as illustrated by FIG. 10.
Next, an explosive well tool such as a tubing cutter 133 illustrated by
FIG. 11 is inserted over the detonator 108 and turned over the threads 95
to the final operational position shown by FIG. 12 with the detonator 108
in ignition proximity with the explosive elements of the tubing cutter
133. The completed assembly is now ready for well placement and
discharge.

[0062] Numerous other modifications and variations may be made of the
structures and methods described and illustrated herein without departing
from the scope and spirit of the invention disclosed. Accordingly, it
should be understood that the embodiments described and illustrated
herein are only representative of the invention and are not to be
considered as limitations upon the invention as hereafter claimed.